Two-element objective for use in the infrared range

ABSTRACT

A two-element objective for operation in the infrared range of 8 Mu and 14 Mu is constituted by a convex-concave front condensing lens of cesium bromide spaced in front of a rear dispersive lens of polycrystalline zinc sulfide fritted under pressure. The objective has a resolving power of 0.8 milliradians on the optical axis with an aperture ratio of 1:2.5.

United States Patent [721 g 'rfi' z' gg a I56] Relerences Cited AM882.999 UNITED STATES PATENTS [22] Filed Dec-8,1969 2.512.257 6/1950Pfund 350/2 5 paemed No 23, 1971 3.035.490 5/l962 Tibbetts 350/2 [73]Agsigneg {m & c 3.439.969 4/1969 Kirkpatrick 350/2 Heidelberg, GermanyFOREIGN PATENTS g5 Pmmy 6-1968 0547.159 10/1951 Canada 350/l64 i P3'13"3 3 OTHER REFERENCES Allen E. Murray. An All lntran DoubletObjective. Ap-

plied Optics. Vol. 4. No. 2. Feb. I965. (350/2) [54] TWO-ELEMENTOBJECTIVE FQR USE m THE Joseph .lerger. Jr.. Infrared Optical Systemsare Design INFRARED RANGE Problems. Optical Spectra. Vol. I. No. 2.April- June I967. 3 1 Drum" Primary Examiner- David Schonberg (52] U.S.Cl. 350/2. mm"! 514mm" Paul Sache' 350/165. 350/195. 350/232Attorney-Waters. Roditi. Schwartz and Nissen [51] lnt.Cl G02b 3/00.

G02 new of Search b gg ABSTRACT: A two-element objective for operationin the infrared range of 8p and I411. is constituted by a convex-concavefront condensing lens of cesium bromide spaced in front of a reardispersive lens of polycrystalline zinc sulfide fritted under pressure.The objective has a resolving power of 0.8 milliradians on the opticalaxis with an aperture ratio of l:2.5.

TWO-ELEMENT OBJECTIVE FOR USE IN THE INFRARED RANGE BRIEF DESCRIPTION OFTHE DRAWING The sole FIGURE of the drawing is a cross-sectional viewtaken through the objective according to the invention.

DESCRIPTION OF THE INVENTION The invention relates to a two-elementobjective for utilization in the mean infrared range, chiefly in thespectral region from 8p. to 14p and which with an aperture ratio ofl:2.5 has a resolving power of about 0.8 milliradians on the opticalaxis.

Image-forming optical systems are already known, functioning in the meaninfrared range. One achromatized, three-element system is composed of acombination of a silicon element, an arsenic trisulfide element andanother silicon element, for operation in the spectral region from 2.7;;to 1 1.0 1.. Another embodiment of an optical system comprises atwoelement objective in which the elements are magnesium fluoride andzinc sulfide chromatically corrected in the spectral region from 3.0;.to 5.0g"

An object of the invention is to provide a chromatically correctedsystem suitable for locational and image-forming functions in theatmosphere from 8p. to I4 According to the invention, the objective isconstructed of a cesium bromide (CsBr) condensing lens I in frontfollowed by a dispersive lens 2 of polycrystalline zinc sulfide (ZnS)fritted under pressure, constituting a two-element achromatic objectivewith air separation.

The materials used for this invention are readily permeable to radiationin the spectral region from 8 to 14 In the arrangement according to theinvention, the lens elements make possible the construction of atwo-element objective achromatically corrected for the stated range.

An embodiment of an objective is next described in detail with referenceto the attached drawing and the following table, in which the objectivehas a focal length of 100 mm. and operates on a wavelength.

mu denotes the refractive index on a wavelength of 10p. and y is theinfrared Abbe index for the spectral region under discussion and isdefined as follows:

where n and n denote the refractive indexes at wave lengths 8 and I4respectively.

Due to the comparatively high solubility of cesium bromide in water itis preferred in the case of an infrared objective according to theinvention (intended for use in the open air) to coat, at least the CsBrsurface exposed to atmospheric action, with a protective film 3 which issubstantially impervious to water but is readily permeable to radiationin the spectral range of operation.

By way of example, such a layer may be a polymerized hydrocarbon such asLupolen l8l0I-I (a branched polyethylene produced by polymerization ofethylene at high pressure) or a polymerized fluorocarbon such as Teflon.

There will be advantage in applying antireflecting interferencecoatings, especially on the highly refractive zinc sulfide lens, and forinstance M4 barium fluoride layers 4 can be em loyed.

. at is claimed is:

1. An achromatic two-lens objective for the infrared range, comprising;a front condensing lens formed of cesium bromide, wherein said lens hasa front surface exposed to the atmosphere and includes a protectivecoating on said front surface impervious to water and permeable toradiation of a wavelength between 8p. and 14p, and a rear dispersivelens constituted of polycrystalline zinc sulfide frittered underpressure spaced from said condensing lens so as to provide airseparation therebetween, the combination of said condensing anddispersive lenses forming an infrared range objective between 8p. and14p. and having a resolving power of about 0.8 milliradians on theoptical axis with an aperture ratio of about 1:25.

2. An objective as claimed in claim 1 wherein said lens has thefollowing data in mm.:

Axial Lens Rndll thickness hou. non.

T1 {-57.40 Front. ii -4.89 1. 00251 80. 2

-a0eu.s

di=2J9 -90.93 Rean. 4,-4.30 2. was 17. 0

wherein:

d is the thickness of the front lens; d, is the spacing of the lenses;

d, is the thickness of the rear lens;

ri is the refractive index at a wavelength of l0;s; and

7, is the Abbe index.

3. An objective as claimed in claim 2 wherein the Abbe index 'y is 10gl"has "14pm wherein:

n and n are the refractive indexes at wavelengths of Bu and 14;:respectively.

i i O i

1. An achromatic two-lens objective for the infrared range, comprising;a front condensing lens formed of cesium bromide, wherein said lens hasa front surface exposed to the atmosphere and includes a protectivecoating on said front surface impervious to water and permeable toradiation of a wavelength between 8 Mu and 14 Mu , and a rear dispersivelens constituted of polycrystalline zinc sulfide frittered underpressure spaced from said condensing lens so as to provide airseparation therebetween, the combination of said condensing anddispersive lenses forming an infrared range objective between 8 Mu and14 Mu and having a resolving power of about 0.8 milliradians on theoptical axis with an aperture ratio of about 1:25.
 2. An objective asclaimed in claim 1 wherein said lens has the following data in mm.: f100
 3. An objective as claimed in claim 2 wherein the Abbe index gamma10 is wherein: n8 and n14 are the refractive indexes at wavelengths of 8and 14 respectively.